The surface tension and the bitter taste intensity of aqueous solutions of propantheline bromide alone and of complexes with alpha-, beta-, and gamma-cyclodextrin have been determined, and a unique correlation between these quantities has been found to hold regardless of the kind and concentration of the cyclodextrin. The surface tension of an aqueous solution of propantheline bromide increases on addition of one of the cyclodextrins, and the effectiveness in surface tension enhancement decreases in the order beta-, gamma-, alpha-cyclodextrin. Analysis of these surface tension data yields the equilibrium constants of 1:1, 1:2, and 2.1 complexations of propantheline bromide and alpha-, beta-, gamma-cyclodextrin. These binding constants are in excellent agreement with those estimated by spectrophotometry. As well as the equimolar complex, beta-cyclodextrin forms the 1:2 complex, and gamma-cyclodextrin forms the 2:1 complex. The dimer of propantheline bromide can be incorporated into y-cyclodextrin. Although the dimer is not included into alpha- and beta-cyclodextrins, the extent of complexation is decreased by the dimerization. The bitter taste of propantheline bromide is suppressed by adding one of the cyclodextrins. The order of this suppression agrees with that of surface tension enhancement. From the relationship between the surface tension and the bitter taste intensity of aqueous solutions of propantheline bromide and the observed value of surface tension for an aqueous solution of propantheline bromide and one of the cyclodextrins, we can predict the bitter taste intensity of the mixed solution. Molecular models of some complexes of propantheline bromide and cyclodextrins have been proposed on the basis of size and shape of these molecules and the affinity of each group of propantheline bromide to the cyclodextrin cavity.